The effect of dextran on subunit exchange of the molecular chaperone αA-crystallin

α-Crystallin, a member of small heat shock protein (sHsp) family, is comprised of αA and αB subunits and acts as a molecular chaperone by interacting with unfolding proteins to prevent their aggregation. The αA-crystallin homopolymer consists of 30–40 subunits that are undergoing dynamic exchange. In vivo, α-crystallin elicits its chaperone action in a crowded cellular environment (e.g. in the lens). In vitro, inert molecular crowding agents (e.g. dextran) are often used to mimic crowded conditions. In this study, it was found that α-crystallin and αA-crystallin are poorer chaperones in the presence of dextran. Using fluorescence resonance energy transfer, it is shown that the αA-crystallin subunit exchange rate strongly increases with temperature. Binding of reduced ovotransferrin to αA-crystallin markedly decreases the rate of subunit exchange, as does the presence of dextran. In addition, in the presence of dextran the effect of reduced ovotransferrin on decreasing the rate of subunit exchange of αA-crystallin is greater than in the absence of dextran. Under the conditions of molecular crowding, the αA-crystallin subunit exchange rate is not temperature-dependent. In the absence of dextran, the exchange rate of αA-crystallin subunits correlates with its chaperone efficiency, i.e. the chaperone ability of αA-crystallin increases with temperature. However in the presence of dextran, the temperature dependence of the chaperone ability of αA-crystallin is eliminated.